FIG. 8 shows a conventional polygonal mirror 1. Usually, a polygonal mirror has a plurality of reflection surfaces 2 on its side surfaces and a through hole 3, for supporting a scanner motor (which is not shown in the figure) to rotate the polygonal mirror, in the center of the polygonal mirror. In making such a polygonal mirror, considerably high degree of flatness for the above described reflection surfaces 2, high closeness to a perfect circle in respect of a radius and an internal surface for the above described through hole 3 which is a reference point for fitting the scanner motor over the through hole 3 are necessary, and further, the distances between the center of through hole 3 and the ridge line for each reflection surfaces 2 must be uniform.
To make a polygonal mirror, such severe requirements must be satisfied, and conventionally, a polygonal mirror is made by applying a cutting process by diamond byte to a metal material such as aluminum, for which accurate cutting process is required and the producing costs are inevitably increased. Further, with a metal polygonal mirror, the increasing request to reduce the weight of a polygonal mirror, which is for reducing the burden laid on the polygonal mirror and reducing the vibration when rotating the polygonal mirror, cannot be obtained.
To solve these defects, a method wherein a polygonal mirror is produced by injection compression molding of synthetic resin has been proposed. Specifically, in this method, an injection molding container is used, and injection compression molding is carried out by injecting melted synthetic resin to a cavity of a metal mold under a low pressure and compressing the synthetic resin in the cavity by a compression core. As shown in FIG. 9, the mold used in this method comprises a pair of lower metal mold 5 and upper metal mold 6, which forms cavity 4 when they are combined. Upper metal mold 6 comprises compression core 7 for compressing the synthetic resin in cavity 4, convex portion 8 to make a through hole in the polygonal mirror is made in the above described lower metal mold 5, and concave portion 9 fitting the above described convex portion 8 when the upper and the lower metal molds are combined is made in the above described compression core 7.
However, when making, by injection compression molding of synthetic resin, a conventional polygonal mirror (shown in FIG. 8) wherein a through hole is made in the center, there are the following problems.
As melted resin injected into the cavity is hardened immediately after it is injected from the regions contacting the surface of the metal mold, that is, the regions shown by oblique lines in FIG. 10, when stuffing of the melted resin into the cavity is completed and the process goes to the stage of compression molding, the surface layer of the stuffed resin is already hardened. If a compression force is applied to the surface layer, excessive stress is caused there. Therefore, overall distribution of the compression force, which is made when the upper and lower metal molds are combined, to the stuffed melted resin is prevented, and the shape of the polygonal mirror becomes different from the desired standard one.
Further, as described above, as a convex portion to make a through hole in the center of the polygonal mirror is made in the center of the cavity, a gate for injecting melted resin into the cavity must be made in the portion off the center of the cavity. Therefore, as the melted resin injected into the cavity is stuffed into the cavity, being divided into two directions because of the convex portion which is in the way, a weld line inevitably occur at the position where the divided melted resin collide. If the weld line occurs on a reflection surface, the degree of flatness of the reflection surface is significantly reduced. Moreover, if a polygonal mirror with a weld line is used, being rotated at high speed for a long time, a crack tends to occur at the position of the weld line, by which reliability is reduced.
As a producing method wherein the occurrence of a weld line on a reflection surface is prevented, the one described in Japanese unexamined patent publication number Sho 63-304223 is known. In this method, pin point gates of the number corresponding to that of reflection surfaces are made radially in a metal mold, and melted resin is injected uniformly from each pin point gate toward a cavity, so a plurality of weld lines are caused at the ridge lines between two, adjacent reflection surfaces. With this method, although the occurrence of a weld line on a reflection surface can be prevented, the occurrence of weld lines themselves cannot be prevented completely, and the problem of reliability in a long-termed use remains as before.
As a producing method wherein the occurrence of weld lines can be prevented completely, the ones described in Japanese unexamined patent publication number Hei 3-304223 or Hei 4-175126 are known. In this method, a gate is provided at the position corresponding to the center of a cavity, and a shaft which also acts as a gate cutter and is once covered with the injected resin is moved up before the melted resin stuffed into the cavity is hardened completely, removing the resin on it, by which a through hole is made. With this method, as the melted resin injected into the cavity is stuffed into the cavity from the center, which corresponds to the position of the gate, being diffused radially, weld lines do not occur in thus made polygonal mirror. However, with this method, as the gate cutter is necessary, the metal mold inevitably becomes complicated and expensive. Moreover, since a through hole is processed when injection molding is almost completed, burrs tend to be caused at the end of the side, which faces the gate, of the through hole, by which the the accuracy of the shape of the aperture is reduced.
Certainly, it is possible to prevent the occurrence of weld lines by changing conditions, such as the temperature of the melted resin injected into the cavity, of the injection compression method, but the range of parameters with which injection compression molding is carried out is narrowed, causing the difficulty of making polygonal mirrors which are faithful to the desired standard one in respect of flatness, shape, size and others.